A.Terry Brennan
responds: With a strong enough contaminant
source, any house can have an indoor air quality
problem. If contaminants are properly controlled, a
tight house can have as clean or cleaner air than a
leaky house.

For example, some of the houses with radon
problems I’ve worked on would need 20 to
30 air changes per hour to get them below the
recommended levels for indoor radon. This could be
done by adding big enough fans, but the house would
cost a fortune to heat and you couldn’t
keep candles lit from the breeze. So the first rule
is "No strong sources of air contaminants in the
house."

Some sources, however, like moisture and odors
from kitchens, laundries, and bathrooms, are
unavoidable. These are actually pretty easy to
control by locally exhausting these rooms. Local
exhaust is better than a general increase in
ventilation because it not only brings in outside
air to dilute the contaminant, it also keeps it
from spreading to the rest of the house. ASHRAE
recommends 100 cfm of intermittent exhaust in the
kitchen and 50 cfm in each bathroom.

Other unavoidable contaminant sources are
scattered throughout the building—notably
people. People give off bioeffluent (body odor) as
well as carbon dioxide and water vapor from
breathing. (With too much CO2 in the air, you feel
drowsy and overheated.) There are also fungi,
bacteria, mites, insects, rodents, dogs, and cats
— who all give off odors — even
in the cleanest of houses. ASHRAE recommends
(Standard 62-1989) that residences have .35 air
changes per hour of general ventilation air to
supply oxygen for breathing and to control
contaminants from these sources.

The ventilation system should be designed to
control the last category of unavoidable
contaminants — soil gases, which include
water vapor and occasionally radon and methane. Use
the air handling equipment to slightly pressurize
the basement or use an exhaust fan to depressurize
the subslab drainage layer. Similarly you should
design the ventilation system to slightly
depressurize the upper parts of the house (in
northern climates) to protect the walls and
ceilings from moisture condensation.